1. Field of the Invention
The invention proceeds from an operating mechanism for a hydraulic actuator in accordance with the preamble of claim 1.
2.Discussion of Background
Operating mechanisms for hydraulic actuators of conventional design are known, which are fitted with plunger coils which are expensive to produce. In addition, the mechanical components belonging to this operating mechanism are comparatively difficult and expensive to produce. An actuator for operating a control valve by means of which, for example, the steam feed to a turbine of a power plant is controlled has a main piston to which, on the one hand, spring force and, on the other hand, pressurized oil are applied. With reducing oil pressure, the spring force reliably closes the control valve, as a result of which the steam feed is interrupted. It is ensured as a result that the turbine does not run out of control should the oil pressure ever fail. The oil pressure in a drive volume which acts on the main piston and operates the control valve via said piston is controlled by a simple electrohydraulic transducer. Given a movement of the control valve in the opening direction, pressurized oil is fed into the drive volume, but because this movement takes place comparatively slowly, comparatively small cross sections are sufficient for feeding the oil. However, a closing movement of the control valve has to take place at a rate which is higher by at least a factor of ten. This causes a comparatively rapid emptying of the drive volume which, however, cannot be achieved by the small cross sections of the oil feed. It is sensible here to use a discharge amplifier which releases correspondingly large cross sections for the oil to flow off after relief.
In addition, it emerges that due to the increase in turbine ratings it is also necessary for the control valves, and thus also the actuators operating them to be designed to be larger and stronger, respectively. A corresponding proportional enlargement of the actuators leads to arrangements with comparatively large amounts of pressurized oil for the purpose of operating them. With commercially available valves, such amounts of oil can be controlled only with great difficulty, added to which the dynamics of the actuator thus also suffer with increasing size.
An operating mechanism for a hydraulic actuator which operates a control valve is known from the document EP-A1-0,430,089. A control loop sets the actuator in accordance with a desired value prescribed by a master plant control and protection system. The discharge amplifier is provided in this case as a plate valve which permits the oil to flow off very rapidly from the drive volume. The plate of the plate valve has at least one cutout which permits cooperation between the pressurized oil in the spring chamber thereof and the oil in the drive volume of the actuator.
Such automatic controls must operate stably in all operating situations, in order to be able to meet the high demands on operational reliability and dynamics. An operating mechanism for a hydraulic actuator which meets these demands can be realized in a conventional way only with a comparatively high outlay.